I'm currently examining the possibilities and implications of linking against a GPL library for an application in a commercial context.

From what I've understood of the GPL, as long as the application is used internally there is no obligation to release its code (even if a copy is moved to a controlled subsidiary).

What I don't understand is the following point from the FAQ :

If a library is released under the GPL (not the LGPL), does that mean that any software which uses it has to be under the GPL or a GPL-compatible license?
Yes, because the software as it is actually run includes the library.

If I take a look at the GPL-compatible licenses, some of them (like the boost one) don't seem to impose the release of the code. Using it would create a situation where you could be compliant with the GPL licence without having to respect its obligation of making your code public (which doesn't seem very credible).

(NB : there are components in Adobe Photoshop licensed under boost and I don't think the code is available on demand)

The most reasonable explication would be that I'm missing something... Could you please tell me where I made a mistake ?

Where did you red that Boost was GPL? It has its own license. The standard rule is that GPLv2 does not mix with proprietary software. There is a question a week on this site about how to get around this. We should ban all GPL questions.
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Andrew FinnellJul 12 '13 at 1:27

I see he said Boost was GPL compatible. The rest of my comment stands.
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Andrew FinnellJul 12 '13 at 1:32

Many of the most common free software licenses, such as the original MIT/X license, ... are "GPL-compatible". That is, their code can be combined with a program under the GPL without conflict (the new combination would have the GPL applied to the whole).

This is a lax, permissive non-copyleft free software license, compatible with the GNU GPL.

Which means that anything licensed under Boost is easily subsumed by the GPL.

Where it gets tricky

Let's say we have project Foo licensed under Boost, and project Bar licensed under GPL and which wants to use Foo.

Bar+Foo is allowed since the licenses are compatible, and the release of Bar+Foo must be GPL as Bar is GPL. Foo, by itself and without BarorBar+Foo, is still available under the Boost license. Said another way, Bar+Foo has no license impact upon Foo itself.

The resulting license of the project combination is a forward acting event for the combination only. It is not a retroactive event.

So if someone else wants to take Foo and do something else with it, they are still free to do so without the copyleft provision of the GPL. However, if they take Bar+Foo, delete Bar and only use +Foo then they are still bound by the terms of the GPL since Bar+Foo was GPL'd.

Your other question:

From what I've understood of the GPL, as long as the application is used internally there is no obligation to release its code (even if a copy is moved to a controlled subsidiary).

The GPL does not require you to release your modified version, or any part of it. You are free to make modifications and use them privately, without ever releasing them. This applies to organizations (including companies), too; an organization can make a modified version and use it internally without ever releasing it outside the organization.

Wholly owned subsidiaries are considered part of the parent organization, so you would legally be in the clear. FSF would point out that you are violating the spirit of Free Software though.

By "GPL-compatible", they mean, a license that does not conflict with the GPL. A less restrictive license, like the Boost license, is "compatible" with the GPL because it doesn't restrict users of the code from doing anything that the GPL would allow a user to do. That is, the Boost license allows people to copy freely, which is what the GPL requires people are allowed.

An example of a license that is not "compatible" would be a license that required a fee to the original authors for users to distribute copies. Since the GPL explicitly permits both free and paid distribution, this more restrictive license would not be compatible.

In other words, the GPL says "You must allow $A, $B and $C". Any license that allows $A, $B, $C and also $D and $E is compatible. A license that allowed $A and $B but not $C would not be.

So the new, combined work is wholly licensed under the GPL, correct? But the original code licensed under MIT can still be distributed separately under MIT without running afoul of the GPL, correct?
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Robert HarveyJul 11 '13 at 5:08

1

IANAL, but yes, that is how it works.
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Fabio FracassiJul 11 '13 at 6:31

4

"because it doesn't restrict users of the code from doing anything that the GPL would allow a user to do" - And also because it doesn't prevent the user from doing anything that the GPL would require them to do.
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Ross PattersonJul 11 '13 at 11:03